Hundreds of vessels were seen clustering near Dubai on Tuesday, as more ships moved away from a still-empty Strait of Hormuz in response to Iran’s efforts to widen its area of control. A ceasefire between the US and Iran has begun to look increasingly fragile, with the two sides exchanging fire even as the US said it had opened a passage through the waterway and CBS reported two American destroyers had crossed into the Persian Gulf. Crew members have reported hearing radio broadcasts warning vessels of new boundaries defended by the Islamic Revolutionary Guard Corps. Attacks on the United Arab Emirates port of Fujairah, meanwhile, underlined an expanded Iranian command zone — and kept the strait largely devoid of traffic in the early hours of the day.